Variations of juvenile abundance of brown shrimp (Farfantepenaeus aztecus) during its main period of migration in Mezquital Inlet, Tamaulipas are presented. Sampling was carried out from 2003 to 2007 between May and July and consisted in the use of an experimental net during the nocturnal period and ebb tide, three days before and after full and new moon. The information was complemented with abundance data of trawl research cruises carried out off the coast of Tamaulipas between May and July from 2003 to 2007. The results showed that the higher values of emigration (kg/night) occurred during the moon period in late May or early June, independently of the lunar phase. The higher values of emigration toward the sea are directly related to the increase in the abundance in the coastal and marine zones. These results are of great importance because the significance of the months of May and June was demonstrated, and can be useful to set the limits of the closed season of the inshore and offshore shrimp fishery.